Sculpted with Multiresolution, enable Dynamic Topology wrecks mesh
The Multiresulition modifier simply does not work with dynamic topology.

When you try to enable dynamic topology with a Multiresulition modifier on the mesh you will get a warning message like this: 


  Warning!
  Generative Modifiers Detected!
  Keeping the modifiers will increase polycount when returning to edit mode.




Also if you hover over the modifier you will see this error message.  


  Not supported in dyntopo




You have to apply the modifier before using dynamic topology.



The only way to "rescue" your sculpt now is to go back to a previous save, before you enabled dynamic topology.